By default, when you start a computer with Windows 8, the default desktop will be the new Metro or Start. But you can start it with the classic desktop as the default one.

To see how you can manually switch between Start(Metro) and Classic desktops on Windows 8 check this tutorial.

Method 1: via Control Panel

1 Go to Control Panel

Press Windows + X keys, then click on Control Panel.(to see other methods to access Control Panel in Windows 8 check this article).

2 Appearance and Personalization

Then go to Appearance and Personalization

3 Taskbar and Navigation

Then click on Taskbar and Navigation

4 Go to Navigation

Click on Navigation tab.

5 Boot directly to classic desktop

Then check the option “When i sign in or close all apps on a screen, go to the desktop instead of Start”.
Then click Apply and Ok.

Next times when you start Windows, the first screen will be the classic desktop instead Start (Metro).

Method 2: From Taskbar

1 Go to classic Desktop

In you are on the Start(Metro) desktop, then press Windows + D keys to go to classic Desktop.
(for other methods to switch between Start(Metro) and classic Desktop check this guide.

2 Right click on Taskbar

Right click on Taskbar, then click Properties.

3 Go to Navigation

Go to Navigation tab.

4 Change the default desktop

Check the option “When i sign in or close all apps on a screen, go to the desktop instead of Start”, then click Apply and Ok.
Next time, when you start computer, the default desktop will be the classic one.